Muwatta Malik 117
موطأ مالك#117Sahihحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ لاَ بَأْسَ أَنْ يُغْتَسَلَ بِفَضْلِ الْمَرْأَةِ مَا لَمْ تَكُنْ حَائِضًا أَوْ جُنُبًا .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "There is no harm in doing ghusl with water that has been used by one's wife as long as she is not menstruating or in a state of major ritual impurity (junub)
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Salim al-HilaliSahih
In-book reference: 2:86